More specifically, the invention relates to a ventilation assembly of the type comprising: a support structure or cowl in which there is defined a conduit having at least first and second axial portions with a larger and a smaller cross-section, respectively, interconnected by an intermediate joining portion, and a motor-driven fan including an impeller mounted rotatable in said conduit and comprising a plurality of blades which extend between a central hub and a peripheral ring. The peripheral ring has an essentially cylindrical main portion to which the radially outer ends of the blades are connected and to which an outwardly diverging end lip is joined. The arrangement being such that between the peripheral ring of the impeller and the wall of the conduit of the support structure or cowl there is defined an annular passage. The annular passage has a first portion defined between the lip of the peripheral ring of the impeller and said joining portion of the conduit and extending in a direction having a radial component, and an essentially axial second portion defined between the cylindrical portion of the peripheral ring of the impeller and the second portion of said conduit.
Ventilation assemblies of this type, according to the prior art, are partially and schematically shown in cross-section in FIGS. 1a to 1d of the accompanying drawings. In these figures the support structure or cowl intended to be fixed to a heat exchanger is indicated by 1, and the impeller of the associated motor-driven fan is denoted overall by 2. The conduit 3 formed in the cowl 1 has two portions 3a and 3b with a larger and a smaller cross-section, respectively, interconnected by an intermediate joining portion 3c (see for example FIGS. 1a and 1b).
The impeller 2 comprises a central hub 4 from which there extends a plurality of blades 5, the radially outer ends of which are connected to a peripheral ring indicated by 6. This ring has an essentially cylindrical main portion 6a to which the blades 5 are connected and to which a radially outwardly diverging end lip 6b is joined. The annular passage defined between the conduit 3 and the ring 6 is denoted by 7.
In the variant according to FIG. 1a the lip 6b of the ring 6 of the impeller extends approximately in the same general plane as the cowl 1, while in the embodiment according to FIG. 1b the lip 6b is set back with respect to said general plane and extends inside the first portion 3a of the conduit 3.
In the variant according to FIG. 1c, the lip 6b of the ring 6 of the impeller extends beyond the conduit 3, at a given distance from the general plane of the cowl 1.
Finally, in the variant according to FIG. 1d, the impeller 2 is positioned, relative to the cowl 1 and to the conduit 3, substantially as in FIG. 1b, but the cowl 1 has an associated anti-recirculation ring 9, known also as an acoustic ring, which extends radially towards the axis of the impeller and is partly facing and in some cases curved towards the impeller 2.
Typically the cowl 1 and the impeller are made of thermoplastic polymers, by means of molding, and their design requires particular attention aimed at reconciling the need for a reduction in weight with that of an acceptable rigidity. The impellers are components which are highly stressed dynamically during operation and their deformations and movements must be suitably taken into consideration in order to avoid contact with stationary parts. For these reasons the gaps between the various parts must be suitably sized. These gaps, which for safety reasons are over-sized, are negatively compensated for in terms of flow rate and pressure of the air flow generated, with an increase in noise, increased electrical power consumption and reduced operating efficiency.